Method for making prestressed column

ABSTRACT

A catheter includes a spring having a plurality of convolutions disposed along a longitudinal axis, and a first modulus. A sheath overlying the spring engages the convolutions of the spring and imparts to the spring axial compressive stresses which increase the modulus of the spring. The catheter can be manufactured by inserting the spring into a tube of Hytrel® material, stretching the tube at a transition region which advances along thhe tube causing the tube to neck down onto the spring. The stretching develops internal stresses which are utimately imparted to the spring thereby increasing the modulus of the catheter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to methods and apparatus forstrengthening columns or elongate tubes, and more specifically forstrengthening spring reinforced catheters and other surgical accessdevices.

2. Discussion of the Prior Art

It is often desirable to increase the strength of elongate columns ortubes, such as tubes formed from flexible plastics. Ordinarily one wouldincrease the wall thickness of the tube in order to provide increasedstrength, but in some cases there are limitations on the maximumdiameter which can be tolerated. Such is the case with medical catheterswhich require the smallest possible outer diameter.

For such catheters, it has been found desirable to form the plastic tubeover a spring thereby increasing the column strength of the catheterwithout sacrificing either flexibility or size. The modulus of such acombination has exceeded the sum of moduli associated with the springand the tubing.

Spring reinforced catheters have been made from several processes all ofwhich require the application of externally generated heat. Mostcommonly, the spring has been inserted into the bore of flexiblepolyvinylchloride tubing which is then heat shrunk so that it collapsesonto the spring. This is a complex process and somewhat restricted as tothe materials which can be used for the tubing.

Coextrusion has also been used to manufacture spring reinforcedcatheters. In this process, the spring is deployed through an extruderas the molten plastic is formed around the spring. Molding processeshave also been used for relatively short lengths of tubing.

In all of these methods of manufacture, heat must be applied to thetubing in significant quantities in order to effectively melt orotherwise shrink the tubing onto the spring. In its melted state, thetubing forms a sheath over the spring which is essentially free ofstress due to the applied heat. This results from the fact that anystresses in the tubing are essentially relieved by the heat. Theresulting structure has a relatively low modulus. Each of these methodsof manufacture requires complex machinery for coextrusion, heatshrinking or molding; in addition, the related processes are extensiveand must be carefully controlled.

Particularly in a catheter construction, the tubing must meet severalrequirements. For the processes of the prior art, it is desirable thatthe material be shrinkable or at least heat formable. It is desirablethat it have a high tensile strength and good flexibility. Complexcatheters, requiring balloons or thermistors or other associatedstructures, also require that the tubing material be solvent bondable.

Polyvinylchloride, polyethylene, urethanes and nylon can all be heatshrunk, but each of these materials fail to meet one or more of theforgoing criteria.

SUMMARY OF THE INVENTION

These shortcomings of the prior art are overcome in the presentinvention by a new process for forming spring reinforced tubing and amaterial particularly adapted to this process and the requirements forcatheter construction.

"Hytrel®" is a trademark of E. I. duPont de Nemours & Co. and is appliedto a material which is solvent bondable, flexible, heat formable, andhas a high tensile strength. Although this material is not particularlyheat shrinkable, it can be longitudinally stretched at normal roomtemperatures without the application of significant heat. By merelyfixing one end of a tube of Hytrel® and grasping the other end of thetube, a tensile stress can be applied to the material which will causethe tube to neck-down, thinning the walls of the tube and decreasing theinternal diameter of the bore. This transformation occurs at a zone oftransition which progresses along the tube as it is stretched. Thisprocess will be referred to herein as "cold extrusion."

It is of particular significance that the stretching of the tubingimparts internal stresses which cause the tubing to shrink slightlythereby imparting the stresses to the internal spring. This results in asubstantial increase in the column strength of the catheter, an increasewhich can be measured in the modulus of the combination.

One aspect of the invention includes a method for making a springreinforced catheter including the steps of providing an elongate tubedefined by a proximal end and a distal end, the tube having an interiorbore with an inside diameter; providing a spring having a proximal end,a distal end, and an outside diameter less than the inside diameter ofthe bore of the tube; inserting the spring at least partially into thebore of the tube; marginally increasing the temperature of the tube at aparticular location along the tube relative to the temperature of theremainder of the tube; and stretching the tube at the particularlocation to reduce the diameter of the interior bore of the tube.

In another aspect of the invention, the method includes the steps ofincreasing the temperature of the tube at a particular location, drawingthe walls of the tube onto the spring at the particular location andexothermically heating the tube at progressive locations along theremainder of the tube.

In still a further aspect of the invention the method includes the stepsof stretching the tube to draw the walls of the tube radially inwardlyinto heat transferring contact with the spring and heating the tube atpositions progressing axially from the particular location.

The resulting spring reinforced catheter includes a spring having alongitudinal configuration and a first modulus in its unreinforcedstate, a sheath overlying the spring and axially compressing the springto a second state wherein the spring has second modulus greater than thefirst modulus.

These and other features and advantages of the invention will be moreapparent with a description of preferred embodiments and reference tothe associated drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side-elevation view of a cold extrudable tube;

FIG. 2 is a side-elevation view of an unreinforced spring;

FIG. 3 is a side-elevation view illustrating the process step ofinserting the spring into the tube in a preferred method of theinvention;

FIG. 4 is a side-elevation view of a process step whereby a portion ofthe tube is cooled with a sprayed fluid;

FIG. 5 is a side-elevation view illustrating a step of wiping the tubewith an evaporative agent;

FIG. 6 is a side-elevation view illustrating the step of heating aportion of the tubing;

FIG. 7 is a side-elevation view illustrating the step of grasping oneend of the tubing and heating the tubing where it is grasped;

FIG. 8 is a side-elevation view illustrating the step of grasping thetubing at the respective ends of the tubing;

FIG. 9 is a side-elevation view illustrating the step of stretching thetubing with the spring at least partially disposed in the bore of thetubing;

FIG. 10 is a side-elevation view illustrating the step of stretching thetubing using a hand thereby causing the tubing to neck-down onto thespring;

FIG. 11 is a side-elevation view illustrating a transition region whichprogresses along the tubing in a preferred process of the invention;

FIG. 12 is a side-elevation view illustrating the internal stresses ofthe tubing which axially compress the spring;

FIG. 13 is an enlarged view of the axially compressed spring of FIG. 12;

FIG. 14 illustrates apparatus for testing the modulus of a springreinforced catheter; and

FIG. 15 is a side elevation view of an exterior tube cold extruded overan interior tube.

DESCRIPTION OF PREFERRED EMBODIMENTS

A tube is illustrated in FIG. 1 and designated generally by thereference numeral 10. The tube 10 can have substantially any dimensionbut for most purposes will have a high aspect ratio such that its lengthis substantially greater than its cross-sectional dimension. The tube 10will typically be cylindrical about an axis 11 and will include walls 12extending radially between an outside diameter and an inside diameter ofthe tube 10. The walls 12 define an inner bore 14 which extends axiallybetween a distal end 16 and a proximal end 18 of the tube 10.

In accordance with a preferred method and embodiment of the invention,the tube 10 is formed from a material which can be cold extruded. Thatis, the ends 16 and 18 of the tube can be separated at room temperaturestretching the tube along the axis 11. During the stretching the walls12 of the tube 10 neck down decreasing both the inside diameter andoutside diameter of the tube. It follows that both the thickness of thewalls 12 and the diameter of the bore 14 are decreased in this processof cold extrusion.

A spring is illustrated generally in FIG. 2 and designated by thereference numeral 21. The spring 21 is formed from a wire 24 typicallyhaving a circular cross-section and being wound into spring convolutions27 adjacent pairs of which may be contacting. The convolutions 27provide the spring 21 with an outside diameter less than the diameter ofthe tube 10, as well as an inside diameter which characterizes a hollowpassage 30 extending axially of the spring 21. The wire 24 willtypically be formed from stainless steel and will have a cross-sectionaldiameter of 0.005 inches. In a preferred method involving catheterconstruction, the tube has an outside diameter of 0.039 inches, and aninside diameter of 0.018 inches; the spring 21 is characterized by anoutside diameter of 0.016 inches and an inside diameter of 0.006 inches.

Materials which can be cold extruded include nylon; however, thismaterial is generally unsuitable for complex catheter constructionbecause it is not solvent bondable. A preferred material is manufacturedby DuPont and sold under the trademark "HYTREL®". This material is notonly cold extrudable but also solvent bondable. It provides a hightensile strength and flexibility which is particularly appreciated incatheter construction. It is also heat deformable. The more commonmaterials found in the art of catheter construction, namelypolyvinylchloride, polyethylene and urethanes, are not particularlysusceptible to cold extrusion and therefore do not benefit as much fromthe present concept.

In a preferred method of manufacture, the tube 10 is axially stretchedcausing the tube to neck down initially at the point of greatestweakness. This point will usually be at a particular location along thetube 10 where the temperature is the highest. In this region the tube 10will first yield to the stresses associated with stretching.

While the process could proceed without regard to the initial positionof that transition region, it may be desirable to dictate that positionby initially locating the transition region at a preferred position suchas the proximal end 16 of the tube 10. In FIG. 4 this particularlocation is designated generally by a bracket 32. It is desirable thatthe temperature of the tube 10 at this particular location 32 berelatively greater than the temperature of the remainder of the tube 10which is designated by the bracket 34.

One way of relatively heating the particular location 32 is to cool theremainder 34 of the tube 10. This can be accomplished by spraying theremainder 34 with cold air 36 from a nozzle 38 as illustrated in FIG. 4.Another way of cooling the remainder 34 of the tube 10 is to wipe thetube in &.hat region with an evaporative agent 41, such as alcohol,using a cloth 43 or other absorbent material. This wiping step isillustrated in FIG. 5. Neither the air 36 illustrated in FIG. 4 nor theevaporative agent 41 illustrated in FIG. 5 is intended to contact theparticular location 32. By thus cooling the remainder 34 of the tube 10,the particular location 32 is relatively heated making this region 32most susceptible to deformation by stretching.

Another way of relatively heating the particular location 32 is toprovide an external heat source such as a current heated wire 45illustrated in FIG. 6. In this case the heat is applied directly to theparticular location 32 and intentionally omitted from the remainder 34of the tube 10.

In a preferred method and apparatus associated with the presentinvention, the tube 10 is formed of Hytrel® material which does notrequire a significant temperature differential between the particularlocation 32 and the remainder 34 of the catheter 10. In fact, if theremainder 34 is maintained at room temperature, the particular location32 can be sufficiently heated by merely grasping the tube 10 between thethumb and index finger of a hand 47. This will impart body heat to theparticular location 32 without raising the temperature of the remainderregion 34. With this process, as illustrated in FIG. 7, the temperatureof the particular location 32 approaches skin temperature of the hand47.

If it is desirable to relatively heat the particular location 32 . . .which is the same as relatively cooling the remainder region 34 . . .some differential in temperature must be achieved. If room temperatureis substantially the same as skin temperature, about 98.6° F., theprocess illustrated in FIG. 7 will not be as effective as the coolingprocesses illustrated in FIGS. 4 and 5 or the heating processillustrated in FIG. 6. It has been found that when the room temperatureis less than 87° F., a suitable temperature differential can beestablished with body heat as illustrated in FIG. 7.

In axially stretching the tube 10, it is desirable that the ends 16 and18 of the tube 10 be held in a device or other holding apparatusillustrated by the arrows 49 and 52 in FIG. 8. These holding apparatus49 and 52 can then be separated to stretch the tube 10. For example, theholding apparatus 49 can be maintained in the fixed location and theholding apparatus 52 moved axially away from the apparatus 49 asillustrated by a pair of arrows 54.

FIG. 9 also illustrates another advantage associated with the preferredmethod. If the spring 21 is merely inserted into the bore 14 and theends 16, 18 of the tube 10 are separated, one will not necessarily knowwhere the spring 21 ends up in the tube. However, if the spring 21 isgrasped or otherwise held along with the end 16 of the tube, itslocation will always be determinable in the final product.

Thus as illustrated in FIG. 10, the hand 47 provides the means forgrasping the distal end 16 of the tube 10, means for heating the distalend of the tube 10, as well as means for retaining the spring 21 in apredetermined location along the tube 10.

As the tube 10 is stretched, it begins to deform in a relatively shorttransition region 54. An enlarged view of this region is presented inFIG. 11 where the region 54 is illustrated to include a first zone A, asecond zone B, and a third zone C. The first zone A is characterized bythe walls of the tube 12 having a temperature (such as room temperature)and being spaced from the spring. The second zone B is characterized bythe tube walls 12 having a second temperature (which may be slightlyhigher than room temperature) and being in substantial contact with thespring 21. The zone C is disposed between zone A and zone B. In zone C,the tube walls -2 are characterized by a third temperature greater thaneither the first temperature associated with zone A or the secondtemperature associated with zone B. The diameter of the walls 12 in zoneC is less than the diameter of the walls in zone A but greater than thediameter of the walls in zone B.

Zone B is disposed from zone A in a particular direction such as thedistal direction of the tube 10. When the tube 10 is drawn axially asillustrated in FIG. 10, the transition zone 54 initially starts in theparticular location 32 and then moves proximally along the tube 10 untilthe entire spring 21 is disposed in zone B.

The temperatures of the respective zones A, B, and C are particularlycritical to an understanding of the cold extrusion process. As the tube10 is initially heated in the particular location 32, the tube firstdeforms in this area as the enlarged tube of zone A transitions throughzone C into contact with the spring 21 in zone B.

As a result of this initial physical deformation, work occurs in anexothermic reaction which heats areas of the tube adjacent to theparticular location 32. This heat which occurs primarily in zone C isgiven up to the spring 21 when the walls 12 contact the spring in zoneB. It follows that the temperature of the walls in zone B will beslightly higher than the room temperature associated with the spring 21.Continued axial tension on the tube 10 will deform the tube at the nextpoint of weakness which will be in the area of the tube which has beenheated by the mechanical exothermic reaction but has not yet passed thatheat to the spring 21. This occurs in the zones A and C of thetransition region 45.

If the tube 10 is grasped at the distal end 16 and drawn distally, aseries of points 56, 58 and 61 disposed proximally along the tube 10will individually and progressively pass through the zones A, C and B(in that order) as the transition region 54 moves proximally along thetube 10.

In a preferred method of manufacture, the Hytrel® tubing is loaded attemperatures below 100° F. and cold extruded or drawn at a rate of about1/10 inch per second. This cold extrusion tends to elongate the tube 10by a factor of three to four while reducing the inside diameter of thetube 10 by a factor of about two.

When the tensile stretching force is stopped, the cold extruded tubing10 tends to relax by as much as 1% to 5% of its length. This commonlyoccurs when tension is applied to any material, as the stretching tendsto develop internal stresses which attempt to draw the material back toits original configuration when the tension is relieved. These internalstresses are of particular importance to the preferred methods andembodiments of the invention. It is these internal stresses which urgethe tube 10 to shorten its length. Were it not for the presence of thespring 21 and the intimate contact between the walls 12 and theconvolutions 27 of the spring 21, the tube 10 would actually exhibit ashortened axial dimension.

In a preferred method wherein the adjacent convolutions 27 of the spring21 are initially contacting, the spring 21 cannot be further compressed,so the internal stresses of the tube 10 are actually transferred to thespring 21 thereby increasing the column strength or modulus of thespring.

In the enlarged view of FIG. 13, the internal stresses are representedby arrows 65. These stresses are communicated through the walls 12 whichmay form a slight ridge 66 between each adjacent pair of theconvolutions 27. These ridges 66 tend to press against the adjacentconvolutions 27 thereby imparting the internal stresses to the spring21.

The internal stresses offer a significant advantage to the presentinvention as can be appreciated with reference to FIG. 13 whichillustrates a typical test for column strength or modulus. An elongatecolumn such as the reinforced spring 21, is laid across two supports 67and 69, and a force P is applied to the object intermediate the supports67 and 69. The amount of deflection which results from the force Pprovides an indication as to the modulus or stiffness of the object.This modulus considers not only the magnitude of the force P and thedistance separating the supports 67, 69, but also the cross-sectionalarea of the object.

The modulus of elasticity for a tube is given by the following FormulaI: ##EQU1## where L is the length between the supports 67, 69;

f is the deflection of the tube; and

P is the force applied to the tube intermediate the supports 67, 69.

Using this formula to calculate the modulus for a 2 Fr. catheter formedof Hytrel® cold extruded over a stainless steel spring, the combinationhaving an outside diameter of 0.027 inches and an inside diameter of0.008 inches, indicates that the modulus of elasticity for thiscombination is 1,240,000 psi. In order to appreciate the significance ofthis figure one would have to test the catheters of the prior art usingthe same formula. Such a test has indicated that polyethylene tubingheat shrunk over the same spring produces a 2 Fr. catheter having amodulus of only 573,000 psi. Thus the cold extrusion process provides amodulus which is more than twice as high as that associated with thecatheters of the prior art. This of course translates into axialstiffness, as well as better pushability and torquability for thecatheter.

If one were to calculate the modulus of the unreinforced tube 10, andthe modulus of the unreinforced spring 21, the prior art which combinesthese two elements would show a modulus which is perhaps 400% greaterthan the sum of the moduli associated with these two components. Thuseven the shrink tubing or coextrusion methods of the prior art providesome increase in strength for the reinforced column. However, with thecold extrusion concept of the present invention, the modulus can beincreased by as much as 800% in order to provide a desired stiffnesswithout sacrificing the increased size of the catheter. Cathetersembodying this concept and having a diameter of only 2 Fr. haveexhibited a modulus greater than 1,000,000 psi.

It will be appreciated that the spring 21 is merely a preferredembodiment of a cylindrical core element that can be prestressed by thecold extrusion of an outer tube 10. In the case of the spring 21, theconvolutions 27 provide a corrugated outer surface which tends toincrease the coefficient of friction between the tube 10 and the spring21. This coefficient of friction can be important in order that theaxially compressing tube 10 does not slip on the spring 21 but ratherengages the spring 21 to axially stress this cylindrical core element.

In a more generic embodiment, this cylindrical core element comprises asecond tube 73 disposed in the bore of the outer sheath or tube 10. Anirregular outer surface 75 can be provided to increase the coefficientof friction between the tube 10 and the element 73. Typically the coreelement 73 will have a modulus greater than the tube 10 in order toprovide maximum stiffness. For example, the core element 73 may beformed of polytetrafluoroethylene and provided with a tubularconfiguration. This embodiment will be of particular advantage where thecatheter requires a smooth inner surface 77.

Although specific preferred embodiments of the concept have beendisclosed, it will be apparent that both the methods and embodiments ofthe invention can be otherwise characterized. Other materials may beapplicable to the cold extrusion process and facilitate the formation ofreinforced springs without expensive heat shrink or coextrusionmachinery. Other methods for heating and cooling particular regions ofthe tube 10 will also be apparent to those skilled in the art. For thesereasons, the scope of the invention should not be ascertained withreference only to the drawings or even the particular embodimentsdescribed, but should be determined only with reference to the followingclaims.

I claim:
 1. A method for making a reinforced catheter comprising thefollowing steps:providing a tube defined by a first end and a secondend, the tube having an interior bore with an inside diameter; providinga core element having a proximal end, a distal end and an outsidediameter less than the inside diameter of the bore of the tube;inserting the core element at least partially into the bore of the tube;increasing the temperature of the tube at a particular location alongthe tube relative to the temperature of the remainder of the tube; andstretching the tube at the particular location to reduce the diameter ofthe interior bore of the tube to a diameter substantially equivalent tothe outside diameter of the core element.
 2. The method recited in claim1 wherein the increasing step includes the step of maintaining theremainder of the tube at room temperature.
 3. The method recited inclaim 1 wherein the increasing step includes the step of heating thetube at the particular location.
 4. The method recited in claim 3wherein during the heating step, the tube is heated to a temperaturegreater than the room temperature and less than about normal bodytemperature.
 5. The method recited in claim 1 wherein the increasingstep includes the step of cooling the remainder of the tube.
 6. Themethod recited in claim 1 wherein the stretching step includes the stepsof:engaging the first end of the tube with a first holder; engaging thesecond end of the tube with a second holder; and after the engagingsteps, separating the first holder from the second holder to stretch thetube.
 7. The method recited in claim 6 wherein at least one of theengaging steps includes the step of grasping the tube.
 8. The methodrecited in claim 7 wherein the one engaging step includes the step ofgrasping the tube at the particular location.
 9. The method recited inclaim 1 wherein the stretching step includes the steps of:cooling thetube at the particular location; exothermically heating the tube at asecond location adjacent the particular location.
 10. The method recitedin claim 1 wherein the core element is a spring.
 11. A method forreinforcing a tube extending between a first end and a second end, andhaving walls disposed along a longitudinal axis which define an internalbore with an inside diameter, the method including the stepsof:inserting into the bore of the tube a core element having an outsidediameter less than the inside diameter of the tube; increasing thetemperature of the tube at a particular location along the tube relativeto the temperature of the remainder of the tube; drawing the walls ofthe tube onto the core element at the particular location; heating thetube at progressive locations along the remainder of the tube; andcooling the tube at the progressive locations along the tube.
 12. Themethod recited in claim 11 wherein the increasing step includes thesteps of:grasping the tube at the particular location; and heating thetube at the particular location to a temperature less than about normalbody temperature.
 13. The method recited in claim 11 wherein the heatingstep includes the step of exothermically heating the tube at theprogressive locations along the tube.
 14. The method recited in claim 12wherein the grasping step includes the step of grasping the tube and thecore element at the particular location.
 15. The method recited in claim11 wherein the cooling step includes the step of moving the walls of thetube into contact with the core element to withdraw heat from the tube.16. The method recited in claim 11 wherein the core element is a spring.17. A method for reinforcing a tube, the tube having walls disposedalong a longitudinal axis and defining an internal bore with an insidediameter, the method including the steps of:inserting into the bore ofthe tube a core element having an outside diameter less than the insidediameter of the tube; increasing the temperature of the tube at aparticular location along the tube relative to the temperature of theremainder of the tube; stretching the tube to draw the walls of the tuberadially inwardly into heat transferring contact with the core element;and heating the walls of the tube at positions progressing axially fromthe particular location.
 18. The method recited in claim 17 wherein thetube is defined by a first end and a second end, and the increasing stepincludes the step of increasing the temperature of the tube at one ofthe first end and the second end relative to the temperature of theremainder of the tube.
 19. The method recited in claim 17 wherein theheating step occurs simultaneously with the stretching step, and thecontinued stretching of the tube draws the walls of the tube at thepositions progressive inwardly into heat transferring contact with thecore element.
 20. The method recited in claim 17 wherein the coreelement is a spring.
 21. A method for reinforcing a tube, the tubehaving a longitudinal axis and an internal bore with an inside diameter,the tube defined by a first end and a second end, the method includingthe steps of:inserting into the bore of the tube a core element havingan outside diameter less than the inside diameter of the tube; heatingthe tube in a transition region including a first zone characterized bytube walls having a first inside diameter greater than the outsidediameter of the core element, and a first temperature, a second zonedisposed in a particular direction from the first zone, the second zonecharacterized by tube walls having a second inside diametersubstantially equivalent to the outside diameter of the core element anda second temperature, and a third zone disposed between the first zoneand the second zone, the third zone characterized by tube walls having athird inside diameter less than the first diameter and greater than thesecond diameter and a third temperature greater than the firsttemperature and the second temperature; and causing the transitionregion to move progressively axially along the tube.
 22. The methodrecited in claim 21 wherein the causing step includes the step ofcausing the transition region to move progressively axially along thetube in a direction generally opposite to the particular direction. 23.The method recited in claim 21 wherein the core element is a spring. 24.A method for increasing the modulus of a column including tubular wallsdefining a longitudinal bore and a core element disposed in the bore,the method including the steps of:engaging the walls of the column;internally stressing the walls of the column; moving the stressed wallsof the column into contact with the core element; releasing the walls ofthe column; and imparting to the core element at least some of theinternal stresses of the walls of the column.
 25. The method recited inclaim 24 wherein during the imparting step the core element is stressedaxially to impart tensile stresses to the core element of the column.26. The method recited in claim 24 wherein the core element is a spring.